The invention relates to television signal transmission systems and methods, and more particularly, to methods and systems for eliminating the effects of NTSC analog television signal interference components on digital advance television (DATV) signals when both are simultaneously transmitted in the same frequency band.
Recent years have witnessed the establishment of a standard for transmission of high definition television (HDTV) signals, over both cable and terrestrial broadcast modes, throughout the United States. Although it offers significantly enhanced picture resolution, terrestrial broadcast of HDTV signals is somewhat problematic due to the almost universally installed base of conventional NTSC broadcast and more particularly, reception equipment. The present system provides for simultaneous transmission (simultaneous broadcast) of HDTV signals and conventional NTSC analog television signals in order to provide high definition television services without rendering the installed base of NTSC receivers obsolete. Conceptually, program material is encoded into the two different formats (NTSC and HDTV) and simultaneously broadcast over respectively 6 MHz transmission channels. Viewers having conventional NTSC equipment would be able of receiving and viewing NTSC programming by tuning in the appropriate NTSC channel, while viewers equipped with HDTV equipment would be able to receive an HDTV program by tuning their receiver to the appropriate HDTV channel. While conceptually simple, simultaneous broadcast of NTSC and HDTV signals often results in characteristic portions of an NTSC signal interfering with adjacent channel or co-channel HDTV signals causing degradation to the HDTV signal.
The cause of this form of signal degradation is well understood by those familiar with high definition television transmission systems and is conventionally termed NTSC co-channel interference. Various means have been proposed in the art to reduce NTSC co-channel interference in current HDTV transmission methodologies, and particularly with respect to vestigial sideband (VSB) HDTV transmissions, which form the basis of the HDTV standard in the United States. Certain of these conventional NTSC interference rejection means are summarized in ATSC standard A/53 (1995) ATSC Digital Television standard. Briefly, the interference rejection properties of a conventional HDTV system are based on the frequency location of the principal components of the NTSC co-channel interference signal within the 6 MHz television channel.
FIG. 1 depicts a typical 6 MHz DTV channel spectrum, represented in baseband in the frequency domain (i.e., symmetric about DC). NTSC co-channel interference is generally recognized as being caused by the three principal carrier components of an NTSC signal; the video carrier, color carrier, and the audio carrier. In the illustrative channel spectrum diagram of FIG. 1, the location and approximate magnitudes of the three principal NTSC components are depicted with the video carrier, indicated at V, located approximately 1.25 MHz from the lower channel bard edge. The color carrier, indicated at C, located approximately 3.58 MHz from above the video carrier frequency, and the audio carrier A is located approximately 4.25 MHz above the video carrier frequency. As depicted in the Figure, and is as well understood in the art, NTSC carrier component interference is of particular concern due to the relatively large amplitudes of the video carrier V and color subcarrier C which characterize NTSC transmission. Although the audio carrier A is presented at a relatively smaller amplitude, it nevertheless contributes a significant interference characteristic. Thus, it will be understood that NTSC co-channel interference rejection is an important consideration in the design of HDTV channels in order to ensure the enhanced quality of an HDTV signal.
Conventional approaches to NOSC co-channel interference rejection are based on the frequency location of the principal components of the NTSC co-channel interference signal within the 6 MHz HDTC channel. FIG. 2A shows a conventional comb filter for NOSC co-channel interference rejection. It, however, has complex circuitry and degrades 3 dB of signal noise rate (SNR). Further, a detector is required to detect NTSC co-channel interference and decide whether to enable/disable NOSC co-channel interference rejection. If NOSC co-channel interference is very small or does not exist the delay lines DL1 and DL2 are bypassed to avoid degradation to the HDTV signal, and thus, its hardware cost is increased. FIG. 2B shows the relationship between signal to noise ratio (SNR) and desired to undesired ratio (DUR) of a digital television receiver with conventional comb filter shown in FIG. 2A. Curve C1 shows the relationship between SNR and DUR when the comb filter in the DTV receiver is disabled. Curve C2 shows the relationship between SNR and DUR when the comb filter in the DTV receiver is enabled. Typically, the comb filter must be disabled when NOSC co-channel interference is very small or not exist, thereby avoiding degradation to the HDTV signal.